The present invention relates to a composition and method for temporarily reducing the permeability of subterranean formations and in particular relates to a composition and method for preventing fluid loss to subterranean formations during well treatments.
At various times during the life of a well, formed in a subterranean formation for the production of oil and gas, it is desirable to treat the well. Such treatments include perforating, gravel packing, fracturing, and acidizing. These treatments generally involve filling the wellbore with a standing or circulating treating fluid. Although high fluid permeability is an important characteristic of a hydrocarbon-producing formation, these treatments may be adversely effected by loss of treating fluid into the highly permeable formations. For example, in an acidizing treatment where it is desired to treat the least permeable formation to improve its permeability, the formation strata having the higher permeability will most likely consume the major portion of the treatment fluid leaving the least permeable formation strata virtually untreated. Therefore it is desired to control the loss of treating fluids to the high permeability formations during such treatments. During fracturing treatments it is desirable to control loss of the treating fluid to formation to maintain a wedging effect and propagate the fracture. During perforating operations, it is desirable to prevent fluids from entering the formation and damaging the formation. During gravel pack operations, it is desirable to recover the expensive workover fluids from the wellbore after completion of the operation. Therefore, the efficient performance of some treatments of the wellbore require temporarily reducing permeability of formation strata to reduce the loss of treating fluid to these formation strata during treatment. Several fluid loss agents have been developed for use in these treatments.
One type of prior fluid loss control agent comprised finely divided solids dispersed in the treating fluid. As the treatment fluid leaked off into the formation, the finely divided solid materials were filtered out onto the face of the formation and formed a filter cake. The filter cake functioned to restrict the flow of fluid therethough and substantially reduce treatment fluid leak off to the formation. Prior types of solid materials included crushed naphthalenes and benzoic acid, crushed oyster shells, silica flower or guar-coated silica flower or crushed limestone and rock salt. The water-soluble materials dissolved during the treatment operation and therefore were unsuitable in many cases. The inert materials formed a solid filter cake which remained on the formation face after completion of the treatment thereby causing, in some cases, permanent damage to the production capability of the formation.
Other types of fluid loss control agents comprised oil-soluble, water-insoluble materials. Prior art materials included soaps, gels, waxes, and various types of polymers of resins such as those disclosed in SPE Papers 5662 (1975) and 5713 (1976) and U.S. Pat. Nos. 3,998,272; 3,979,304; 3,979,305; 3,989,632; 3,882,029; 3,181,612; 3,252,904, 3,336,979 and 3,319,716 which discloses oil soluble polyesters. These materials were mixed with the treatment fluid and injected into the wellbore. These materials also formed a filter cake on the face of the formation to prevent treatment fluid leakoff. It was intended that these materials be dissolved in the subterranean hydrocarbon fluids as the temperature of the formation increases after the treatment operation. Although these materials were suitable in some applications, in fracturing treatments and treatment of gas wells, these materials, in some cases, left a residue on the face of the formation after completion of the treatment thereby causing permanent damage to the production capabilities of the formation.
Water soluble and acid degradable polymeric fluid loss control agents have also been utilized in the prior art as disclosed in U.S. Pat. No. 3,319,716 and SPE Papers 3653 (1971). A field practice of removing these materials after treatment involved acidizing the formation after treatment. Although these materials were suitable in some applications, in those cases where there was incomplete acid contact with the fluid loss control material, these materials also left a residue in the formation after treatment thereby damaging the production capabilities of the well.
Another type of fluid loss control agent known as ball sealers have been developed for use during diversion treatments in cased perforated wellbores. The diversion treatments generally comprise diverting the treatment fluid from the more permeable formation strata to the least permeable formation strata to enable treatment of the least permeable formation strata. The ball sealers, which generally comprise small, rubber-coated balls, were suspended in the treating fluid and pumped into the well along with the treating fluid. The balls were carried down the interior of the casing to the perforations communicating with the high permeability formation strata. The ball sealers seated on these perforations and diverted the treating fluid to a formation strata having a lower permeability. In some applications, the presence of the ball sealers in the wellbore after the treatment presented operational problems.
Therefore, there is still a need for a fluid loss control diverting agent which can effectively prevent fluid loss during treatment operations and is capable of being removed from the well after treatment operations without leaving any residue in the wellbore or on the formation strata.